On February 7, 1992, on the edge of a small town in Louisiana, a six-year-old-boy knocked on the door of a neighbour’s house to see if the children who lived there wanted to play. The door was opened by the lodger — a convicted child molester called Ricky Langley. Langley took the boy, Jeremy Guillory, upstairs to his own room, probably sexually abused him, then strangled him to death and hid him in the closet. Three days later Langley led police to the body.

More than a decade later Langley was facing his second trial, this one potentially ending in the death penalty, and his case was being supported by the celebrated anti-death penalty lawyer Clive Stafford Smith and a team from the Louisiana Capital…